This invention relates to a nipped roller impression sensor system, and more particularly to a system employing a pressure sensor device for insertion between nipped rollers.
Nipped rollers, i.e.xe2x80x94two rollers that operate in contact with each other such as illustrated in FIG. 1, are used in many industries. For instance, in the paper making and converting industries, nipped rollers are used to process paper by removing water from the sheet and providing a desired finish to the paper. In order to do this, a constant and desired amount of pressure must be maintained between the nipped rollers so as to impart desired properties to the sheets without damaging them. Some of the consequences of failing to maintain a constant and desired amount of pressure between the rollers are sheet wrinkles, excessive sheet breaks, damaged rollers, etc. As such, paper mill operators routinely need to measure the pressure exerted by the nipped rollers, and if necessary, adjust the position of the rollers so as to reacquire a desired amount of pressure therebetween.
The prior art teaches a variety of ways to measure pressure between a pair of nipped rollers so as to determine the width of a nip. One approach used to sense the uniformity of the pressure between nip rollers is disclosed in U.S. Pat. No. 5,562,027 to Moore, entitled xe2x80x9cDynamic Nip Pressure and Temperature Sensing Systemxe2x80x9d, which discloses a system that comprises a roller having sensors disposed thereon for measuring pressure at several locations along the roll length, wherein the measurements obtained by the system are transmitted to a computer and display, and in one embodiment, corrective measures are initiated. The pressure sensors, as disclosed, can be piezoelectric, piezoresistive, strain gauges, etc. and are mounted directly onto the rollers. Thus, once during each revolution of the roller, the sensor is positioned between the rollers and thus experiences pressure between the rollers, whereby the sensor transmits a signal representing the amount of pressure experienced to the computer and display.
Another approach used in the prior art to measure the width of a nip between a pair of rollers is to pass a sheet of carbon paper on a white backing material between the rollers, and to analyze the impression or resulting mark. FIG. 4 illustrates a sample of an impression made by the carbon paper procedure. However, for several reasons, this procedure is inadequate to measure the width of the nip between the rollers.
First, this procedure cannot be performed while the machine is in operation. The machine must be stopped so that the carbon paper may be placed between the rollers when the rollers are in the open position (i.e.xe2x80x94when the rollers are not in contact with each other). The rollers are then brought into contact with each other and the pressure between the rollers causes the carbon paper to leave an impression on the backing paper. The carbon paper cannot be positioned between the rollers while the rollers have paper passing therebetween, since the paper would not be adequately pressed when the rollers are opened to insert the carbon paper. Thus, the procedure is typically only performed during machine down-times (e.g.xe2x80x94when the machine is out of service due to breakdowns or service interruptions).
Additionally, the method of taking carbon paper impressions is inadequate when three or more rollers are disposed consecutively, as shown in FIG. 2. This follows because an adjustment made to the position of one of the three or more rollers causes the pressure to change between each other pair of rollers. Furthermore, if an impression has been made on carbon paper between one pair of rollers and the rollers are adjusted so as to reduce the pressure therebetween, a new sheet of carbon paper needs to be inserted between all of the pairs of rollers. The lower pressures between each pair of rollers would result in narrower markings, which would be obscured by the wider markings taken at the higher pressure. Thus, adjusting three or more rollers is very time-consuming and labor intensive, since every roller adjustment made would have an unintended effect on the pressure between other pairs of rollers, requiring that new carbon paper impressions to be taken.
Also, the time required to perform the analysis of the carbon paper impressions is undesirably lengthy. Because of this analysis time, corrective action that needs to be taken (i.e.xe2x80x94adjustments made to the positioning of the rollers) is often delayed until a next down-time, rather than the down-time when the pressure was taken. This causes the quality of the rolled product to suffer, since the position of the rollers could be adjusted immediately if the time for analysis was shorter.
In an attempt to improve upon the method of taking carbon paper impressions, force sensing resistors are employed in the prior art in order to measure an amount of force between nipped rollers. FIG. 5 illustrates one example of a force sensing resistorxe2x80x94a linear potentiometerxe2x80x94having two polymer films or sheets. On one sheet, a conducting pattern is deposited on the polymer in the form of interdigiting electrodes 2. Semi-conducting polymer 1, which is a pressure-responsive resistive layer having a force/resistance characteristic such as illustrated in the graph of FIG. 6, is deposited on the other sheet. The pressure-responsive resistive layer, as shown in FIG. 6, has a resistance that decreases as the pressure on the layer increases. When the two sheets of the force sensing resistor are brought together but no pressure is exerted thereon, the resistance between the interdigiting electrodes is very high. However, when the force sensing resistor is inserted between the rollers and pressure is exerted thereon, the resistance of semi-conducting polymer 1 reduces sufficiently to shunt an electrical conduction at the interdigiting electrodes where the pressure is exerted.
One problem with traditional force sensing resistors in the prior art, however, is that they are inaccurate. For instance, a nip pressure applied to a prior art sensor may inadvertently shunt an electrical conduction through an area of the force sensing layer that exceeds the particular area where pressure is applied. This occurs because, when a pressure is applied to the force sensing layer, the force is spread over the force sensing layer. This problem is worsened if an overlay or laminating surface is used, since the force is further spread out over the force sensing layer. Traditional force sensing resistors in the prior art also require substantial calibration, and must account for such variables as temperature, humidity, and aging. In addition, the pressure-responsive resistive layer is expensive to use.
Thus, there is a need for an improved and inexpensive system for measuring the width (or impression) and the position of a nip accurately and conveniently without the need for calibration.
According to one embodiment, the present invention is directed to a nipped roller impression sensor system comprising a first roller having an axis of rotation and a surface and a second roller having an axis of rotation that is substantially parallel to the axis of rotation of the first roller, wherein the second roller is disposed so that a surface of the second roller exerts pressure on the surface of the first roller at a nip having a width. The nipped roller impression sensor system also comprises at least one pressure sensor device configured to be disposed at the nip between the first and second rollers. The pressure sensor device comprises a resistive electrode maintained in a non-conductive arrangement with a shunt electrode by at least one spacer element, such that a width of the nip corresponds to a change in resistance across the resistive electrode when the second roller exerts pressure on the first roller.
According to another embodiment, a position of the nip corresponds to a change in resistance between the shunt electrode and a location along the resistive electrode. According to another embodiment, the system further comprises a means for measuring a change in resistance across the resistive electrode and a change in resistance between the shunt electrode and a location along the resistive electrode. According to another embodiment, the system further comprises a processor for calculating the width and position of the nip. According to another embodiment, the impression sensor system comprises two pressure sensor devices, and the processor is configured to calculate a width and a position of the nip at each of the pressure sensor devices. According to another embodiment, the system is further configured to adjust the calculated nip width if a position of the nip measured by a first pressure sensor device differs from the position of the nip measured by a second pressure sensor device. According to another embodiment, the adjusted nip width corresponds to the angle of the pressure sensor device relative to the rollers.